When organic substrates, in particular those having groups capable of reacting with isocyanate end groups, are coated with polyvinyl chloride (PVC) plastisols or with melts of soft PVC, the bond strength of the coating is often insufficient for practical purposes. So-called bonding agents are therefore used to improve the bond strength (see Beschichten mit Kunststoffen, Carl Hanser Verlag, Munich 1967, and Kunststoffe, 68, (1978), page 735 et seq and page 800 et seq).
The coating of textile sheet products, in particular those based on polyamides (e.g. Nylon 6, Nylon 66 or aramides) or polyesters (in particular polyethylene terephthalates or polybutylene terephthalates) cannot be carried out satisfactorily with PVC plastisols alone because the strength of the bond obtained is insufficient. For this reason, a primer coating based on PVC plastisol containing isocyanate is first applied to the textile sheet. This primer coat develops a firm bond to the textile substrate and to the subsequently applied layers of pure PVC.
Suitable isocyanate bonding agents for this purpose, include, for example, the solution of isocyanato isocyanurates in plasticizers described in German Auslegeschrift No. 2,419,016. Suitable bonding agents have also been described in German Offenlegungsschrift No. 3,041,732. When such one-component bonding agents are mixed with plastisol, the isocyanates are found to react with traces of water as well as with other reaction components or catalytically active substances present in the plastisol mixture (e.g. metal stabilizers for PVC).
Apart from these one-component bonding agents (containing only polyisocyanates), two-component bonding agents composed of hydroxyl polyesters and polyisocyanates as solutions or in solid form have been in use for many years (see e.g. German Pat. No. 957,117 and German Auslegeschrift No. 1,719,123).
The main disadvantage of all these systems, however, is the limited storage stability of the PVC plastisols which contain these polyisocyanate bonding additives. Due to the above mentioned reactions of the isocyanates as well as reactions with atmospheric moisture, undesirable increases in viscosity occur which limit the storage stability of such mixtures even at room temperature.
It was therefore an object of the present invention to provide new bonding agents for PVC plastisols so that PVC plastisols containing polyisocyanate-based bonding agents could be obtained with a very high stability in storage but with the capacity for excellent adherence when heat cured.